Manipulating A SqlDatSource UpdateCommand In Code-behind

We've got an employee database that I'm modifying to include two photos of each employee, a small thumbnail image and a full-size image.  The HR department maintenance page contains a listbox of employee names, which, when clicked, populates a detailsview control.
To get the images to display and be updatable, I've had to structure the following SqlDatasource and DetailsView:

1    <asp:DetailsView ID="dvEmp" runat="server"
2      AutoGenerateRows="false"
3      DataSourceID="dsEmpView"
4      DataKeyNames="empID">
5    <Fields>
6    <asp:CommandField ShowEditButton="true" ShowCancelButton="true" ShowInsertButton="true" />
7    <asp:BoundField HeaderText="Name (Last, First)" DataField="empname" />
8    <asp:TemplateField HeaderText="Thumbnail photo">
9    <ItemTemplate>
10   <asp:Image ID="imgThumbnail" runat="server" ImageUrl='<%# formatThumbURL(DataBinder.Eval(Container.DataItem,"empID")) %>' />
11   </ItemTemplate>
12   <EditItemTemplate>
13   <asp:Image ID="imgThumbHidden" runat="server" ImageUrl='<%# Bind("thumbURL") %>' Visible="false" />
14   <asp:FileUpload ID="upldThumbnail" runat="server" />
15   </EditItemTemplate>
16   </asp:TemplateField>
17   <asp:TemplateField HeaderText="Full Photo">
18   <ItemTemplate>
19   <asp:Image ID="imgPhoto" runat="server" ImageUrl='<%# formatImageURL(DataBinder.Eval(Container.DataItem,"empID")) %>'  />
20   </ItemTemplate>
21   <EditItemTemplate>
22   <asp:Image ID="imgPhotoHidden" runat="server" ImageUrl='<%# Bind("photoURL") %>' Visible="false" />
23   <asp:FileUpload ID="upldPhoto" runat="server" />
24   </EditItemTemplate>
25   </asp:TemplateField>
26   </Fields>
27   </asp:DetailsView>
28  
29   <asp:SqlDataSource ID="dsEmpView"
30     runat="server"
31     ConnectionString="<%$ ConnectionStrings:eSignInConnectionString %>"
32     OnInserting="dsEmpView_Inserting"
33     OnUpdating="dsEmpView_Updating"
34     SelectCommand="SELECT empID, empname, photoURL, thumbURL FROM employees where (empID = @empID)"
35     InsertCommand="INSERT INTO employees (empname, photoURL, thumbURL) values(@empname, @photoURL, @thumbURL)"
36     UpdateCommand="UPDATE employees SET empname=@empname, photoURL=@photoURL, thumbURL=@thumbURL WHERE (empID = @empID)">
37     <SelectParameters>
38       <asp:ControlParameter ControlID="lbxEmps" Name="empID" PropertyName="SelectedValue" Type="Int16" />
39     </SelectParameters>
40   </asp:SqlDataSource>
41  
42   -----
43  
44   Protected Sub dsEmpView_Updating(ByVal sender As Object, ByVal e As SqlDataSourceCommandEventArgs)
45     Dim bAbort As Boolean = False
46     Dim bThumb(), bPhoto() As Byte
47     If e.Command.Parameters("@ename").Value.trim = "" Then bAbort = True
48     Dim imgT As FileUpload = CType(dvEmp.FindControl("upldThumbnail"), FileUpload)
49     If imgT.HasFile Then
50       Using reader As BinaryReader = New BinaryReader(imgT.PostedFile.InputStream)
51         bThumb = reader.ReadBytes(imgT.PostedFile.ContentLength)
52         e.Command.Parameters("@thumbURL").Value = bThumb
53       End Using
54     End If
55     Dim imgP As FileUpload = CType(dvEmp.FindControl("upldPhoto"), FileUpload)
56     If imgP.HasFile Then
57       Using reader As BinaryReader = New BinaryReader(imgP.PostedFile.InputStream)
58         bPhoto = reader.ReadBytes(imgP.PostedFile.ContentLength)
59         e.Command.Parameters("@photoURL").Value = bPhoto
60       End Using
61     End If
62     e.Cancel = bAbort
63   End Sub

If the user updates both images at the same time by populating their respective FileUpload boxes, everything works as advertized.  But if the user only updates one image (or neither image), things break. If they upload, say, just the full-size photo during an update, then it gives the error "System.Data.SqlClient.SqlException: Operand type clash: nvarchar is incompatible with image".I think this error occurs because the update command is trying to set the parameter "thumbURL" without having any actual data to set.  But since I really don't want this image updated with nothing, thereby erasing
the photo already in the database, I'd rather remove this parameter from the update string.So, let's remove the parameter that updates that image by adding the following code just after the "End Using" lines:
 Else
Dim p As SqlClient.SqlParameter = New SqlClient.SqlParameter("@thumbURL", SqlDbType.Image)
e.Command.Parameters.Remove(p)
(Similar code goes into the code block that handles the photo upload)Running the same update without an image in the thumb fileupload box, I now get this error: "System.ArgumentException: Attempted to remove an SqlParameter that is not contained by this SqlParameterCollection."Huh?  It's not there?  Okay, so lets work it from the other end: let's remove all references to the thumbURL and photoURL from the dsEmpView datasource.  We'll make its UpdateCommand = "UPDATE employees SET empname=@empname WHERE (empID = @empID)", and put code in the
dsEmpView_Updating sub that adds the correct parameter to the update command, but only if the fileupload box has something in it.  Therefore: If imgT.HasFile Then
Using reader As BinaryReader = New BinaryReader(imgT.PostedFile.InputStream)
bThumb = reader.ReadBytes(imgT.PostedFile.ContentLength)
e.Command.Parameters.Add(New SqlClient.SqlParameter("@thumbURL", SqlDbType.Image, imgT.PostedFile.ContentLength))
e.Command.Parameters("@thumbURL").Value = bThumb
End Using
End If

(Similar code goes into the code block that handles the photo upload)But reversing the angle of attack only reverses the error. Uploading only the photo and not the thumb image results in: "System.Data.SqlClient.SqlException: The variable name '@photoURL' has already been declared. Variable names must be unique within a query batch or stored procedure."So now it's telling me the parameter IS there, even though I just removed it.ARRRGH!What am I doing wrong, and more importantly, how can I fix it?Thanks in advance.